Microelectronic security coatings

ABSTRACT

A security coating on an electronic circuit assembly comprises a mesh coating that may have a unique signature pattern and comprise materials that easily produce an image of the signature so that it is possible to determine if reverse engineering has been attempted. Spaces in the mesh may include electrical components to erase circuit codes to destroy the functionality and value of the protected die if the mesh coated is disturbed. The voids may include compositions to enhance the mesh signature and abrade the circuit if tampering takes place.

BACKGROUND

[0001] This invention relates to techniques for applying securitycoatings to microelectronic circuits.

[0002] Coatings are applied to microelectronic circuits to restrictreverse engineering of the circuit layout and deny access to embeddedcodes. Coatings can provide a physical barrier that destroys theunderlying components when the coating is manipulated in some fashion,e.g. cut or tampered with. Current techniques, in general, involveapplying one or more single-layer or single-component coatings, e.g. asdepicted in FIG. 2, where a primer 10 and a protective coating 2 cover acircuit set or die 14. Coatings may also provide a protective barrier tocertain forms of electromagnetic inspection.

[0003] Despite significant strides in this area, among them the use ofthermal spray to apply coatings described in U.S. Pat. No. 6,319,740,6,287,985, 6,110,537, 5,877,093 and 5,762,711, industry desires highersecurity as more valuable information is stored in electronic circuitsthat are increasingly used to support the economic and sustaininginfrastructures of the world. The higher value information resident inelectronic circuits and their designs raises the risk of unauthorizedreverse engineering of the protected system and a resultant loss ofvaluable information through unauthorized duplication, spoofing and useof such “target devices”, any electronic system that contains componentsthat require protection from physical, chemical, electrical, acoustic orspectral methods of inspection.

SUMMARY

[0004] An object of the present invention is to provide superiorsecurity coatings for target devices.

[0005] According to the invention, instead of applying asingle-component coating as a protective coating or base layer for othercoatings, a mesh layer is applied, with which other coatings or featurescan be applied or inserted. Mesh not only improves the mechanicalproperties between a single component coating, the device and othersecurity layers and components, it provides a barrier that is difficultto penetrate with invasive and non-invasive inspection processes thatare often used to reverse engineer the underlying circuitry and accesscritical, embedded codes.

[0006] According to one aspect of the invention the mesh coating isapplied randomly by hand or a programmed applicator, e.g. using Computerassisted design (CAD) deposition wherein the beaded mesh pattern ispredetermined and is robotically, site directed to enable a uniquepattern of deposition for each coated device or batch of devices.

[0007] According to one aspect of the invention, the mesh coating is apreformed sheet.

[0008] According to one aspect of the invention individual mesh elementsare created according to a pre-determined or randomized pattern toproduce what may be called a distinctive “signature” mesh pattern thatcan be read by a non-invasive inspection method. Attempts to tamper withthe protected circuit will alter the signature, which can be detected byinspecting the mesh.

[0009] According to one aspect of the invention, an active device, suchas a piezoelectric transducer, is inserted between one or more of themesh elements. If the mesh is disturbed, the transducer output triggersan instruction set to the circuit to initiate an erase or corruptsequence of the valued code in the critical software physical domain ofthe target device.

[0010] According to one aspect of the invention, abrasive materials areinserted in the mesh that physically damage the circuit when the mesh isdisturbed.

[0011] According to another aspect of the invention, the mesh coatingmay be applied at another layer level of the security coating to providea signature pattern for the coating.

[0012] According to one aspect of the invention, safe levels ofalpha-emitting materials are incorporated into the mesh, providing asignature emission pattern or a serve as the basis of an embeddedsensor.

[0013] According to one aspect of the invention the opened regionsformed by the mesh are filled with a material different in compositionand physical features from the mesh material that absorb, reflect, ordiffuse acoustic and electromagnetic radiation to degrade the circuitimage obtained using electromagnetic radiation imaging devices.

[0014] According to one aspect of the invention the opened regionsformed by the mesh are filled with a material different in compositionand physical features from the mesh material that enhance the signaturecharacter of the mesh coating.

[0015] Other objects, aspects, features, and benefits of the inventionwill be apparent from the description and drawings

BRIEF DESCRIPTION OF THE DRAWING

[0016]FIG. 1 shows a die that has been coded according to the prior art.

[0017]FIG. 2 shows a die coated according to the invention, using a meshand barrier coating.

[0018]FIG. 3 shows a die covered by a mesh wherein the mesh spaces arefilled with an electronic device.

[0019]FIG. 4 is a typical plan view of a mesh pattern.

[0020]FIG. 5 is a perspective showing a mesh overlaying a die

[0021]FIG. 6 shows a computer assisted design deposition of the meshpattern.

[0022]FIG. 7 shows different ways to apply a mesh to an uncoatedintegrated circuit or multichip module to produce different securitycharacteristics.

DESCRIPTION

[0023] In contrast with the prior art coating shown in FIG. 1, FIGS. 2and 3 demonstrate the use of a mesh coating 16 on a circuit or die 14.In FIG. 2, a protective coating 12 is applied that fills the spaces oravoids 16 a in the mesh, shown best in FIG. 4. In FIG. 3, where theprotective coating 12 is also used, the voids 16 a first are filled withan active electrical device 19 that produces, when disturbed, anelectrical signal initiating a software instruction for erasing orcorrupting all or part of stored, critical code imbedded in the circuit14. This active device 19, in effect a sensor, may be a chargedcapacitor (powered component) or a piezoelectric transducer (unpoweredcomponent). A protective coating like the coating 12 in FIG. 1 can beapplied over the mesh using a thermal spray method as explained thepatents enumerated above. The mesh coating can also be modified toinclude multiple layers of meshed coatings.

[0024] Referring to FIG. 5, the mesh 16 may have three-dimensionalcharacteristics; that is, the mesh is slightly elevated at points andmay or may not be uniform in standoff height from the die. The patternof the individual mesh elements 16 b can follow a random or uniformpattern that results in differences in opacity, refractive index,density, hardness, molecular weight, atomic weight, dielectric constant,chemical reactivity and thermal conductivity across the mesh coating. Asa result the appearance of the circuit from above the mesh coating isoccluded. Thus, a meaningful image of the underlying die layout isobstructed for non-invasive and invasive inspection methods. Moreover,slicing away the mesh, an invasive inspection, will destroy portions ofthe die where the mesh is attached, scrambling the circuitconfiguration.

[0025] Different ways to apply the mesh are shown in FIG. 7. The meshmay be a preformed material sheet 20 that is prepared to the proper sizeto cover just a die 23 or the full cavity 25 of the target device 27 (anuncoated integrated circuit or multichip module). The mesh may also bepainted-on or sprayed-on in a random or signature (unique identifying)pattern using a CAD-controlled auto-dispenser 17. The preformed mesh 20can also be a signature coating.

[0026] In some applications, a monolithic pre-coat 30 is applied to thedevice 27 which is followed by a mesh topcoat to produce the coatingconfiguration 32, where any tampering with the mesh top coat could bedetected as a change in the signature imbedded in the mesh layout.

[0027] Coating configurations 34, 36, 38 show different ways to apply anovercoat on the mesh for die-only and full or partial microelectronicassemblies (e.g. multichip modules). In configurations 34 and 36, theresult is a composite mesh layer with an integrated void-fillingsecondary coating. The difference between the two is that in arrangement34 the entire device 27 is coated; in the arrangement 36 just the die iscoated. In coating configurations 38, the overcoat covers the entiremesh 16 and the mesh elements 16 b can be marker-based, as explainedbelow, to provide a hidden undercoat signature, in addition to thesignature of the mesh pattern.

[0028] The mesh can be constructed from a single bead or from pre-formedcomponents that may be either as a single organic or inorganic resinmaterial or as a composite blend of resin with filler. The meshcomposition should yield an adherent and coherent mixture that can befully cured to a hardened mesh pattern (or cure as the material isdeposited). The mesh material may be capable of being B-staged,permitting interaction with other coating layers and subsequent curing,yielding a hardened mesh pattern that is fused to the circuit and theovercoat coatings. The mesh coating can include a UV-fluorescent die ornon-toxic levels of an alpha emitter (e.g. ²⁴¹Am) that are incorporatedinto predetermined strands or banded patterns, producing a distinctiveidentifying signature pattern under light activation or by ascintillator. Materials uniquely detectable by short wave and long waveview the light or by other electromagnetic radiation detection methodsmay be selectively incorporated into individual mesh strands for thesame purpose. Safe levels of particle-emitting materials may included inthe mesh (16). The signature can be imaged from such energy emissions.

[0029] The spaces 16 a in the mesh may be filled with highly abrasiveparticle compositions to obstruct access to critical or sensitivecomputer program device design information. Filler materials damage thecritical elements of the target device if attempts to slice ormechanically remove the mesh 16 are made. Some or all of the spaces 16 acan be filled with materials that absorb, reflect, or diffuse acousticand electromagnetic radiation to degrade the circuit image obtainedusing non-intrusive imaging methods. The mesh or a material in thespaces can comprise safe levels of energy emitting materials, such asalpha particles, that can be externally detected to produce an image ofthe mesh signature or its outline.

[0030] One skilled in the art may make modifications, in whole or inpart, to an embodiment of the invention and its various functions andcomponents without departing from the true scope and spirit of theinvention.

1. A method of coating a circuit comprising: applying a first material as a mesh on the circuit; and applying a coating material over the mesh.
 2. The method described in claim 1, wherein the mesh is a sheet of said material.
 3. The method described in claim 1, wherein the mesh comprises a signature pattern.
 4. The method described in claim 1, further comprising inserting an abrasive material into a space in the mesh.
 5. The method described in claim 1, further comprising: inserting a sensor component in a space in the mesh material, said component comprising means for providing a signal to the circuit when the component is disturbed.
 6. The method described in claim 1 wherein the mesh material comprises UV-fluorescent materials.
 7. The method described in claim 1, wherein the mesh material comprises emissive materials.
 8. The method described in claim 1 wherein the mesh comprises a material that attenuates electromagnetic energy.
 9. The circuit described in claim 1 wherein the mesh comprises strands of different materials.
 10. The method described in claim 2, wherein the mesh provides a signature pattern.
 11. The method described in claim 2, further comprising inserting an abrasive material into a space in the mesh.
 12. The method described in claim 2, further comprising inserting a sensor component in a space in the mesh material, said component comprising means for providing a signal to the circuit when the component is disturbed.
 13. The method described in claim 3, further comprising inserting a hard, abrasive material into a space in the mesh.
 14. The method described in claim 3, wherein a sensor component is inserted into at least some of the mesh material, said component comprising means for providing a signal to circuit when disturbed.
 15. A method for determining if a circuit has been tampered with comprising: applying a first coating to the circuit in a signature pattern; applying a second coating over the first coating; producing an image of the first coating; and examining the image for the characteristics of the signature pattern.
 16. The method of claim 15, wherein the first coating comprises a mesh.
 17. The method of claim 16, where the mesh comprises a material that produces energy and said image is produced from said energy.
 18. The method of claim 15, further comprising applying energy to the circuit to produce said image.
 19. The method described in claim 15, wherein mesh strands comprise different materials to produce a change in the image of the mesh.
 20. A method for determining if a circuit has been tampered with comprising: applying a mesh in a pattern as the final topcoat to the circuit in a signature pattern, said pattern defining a signature; and producing an image of the mesh pattern and comparing it with said signature, any differences indicating that the mesh has been disturbed.
 21. A combination comprising: a circuit; a mesh coating over the circuit; and an abrasive material in a space in the mesh.
 22. A combination comprising: a circuit; a mesh coating over the circuit; and an electrical component in a space in the mesh to provide a signal to the circuit when the component is disturbed.
 23. A combination comprising: a circuit; and a mesh coating over the circuit comprising an energy emitting material.
 24. A combination comprising: a circuit; a mesh coating over the circuit comprising an energy absorbing material.
 25. A combination comprising: a circuit; a mesh coating over the circuit that has a signature pattern.
 26. The combination of claim 25, wherein the mesh coating comprises an energy absorbing material.
 27. The combination of claim 25, wherein the mesh coating comprises an energy emitting material. 